The microscopic mechanism of platinum cluster nucleation on DNA templates is studied by first-principle molecular dynamics simulations. We find that Pt(II) complexes bound to DNA can form strong Pt-Pt bonds with free Pt complexes after a single reduction step, and may thus act as preferential nucleation sites. This is confirmed by a series of reduction experiments, in which we achieve purely heterogeneous platinum growth on DNA, and use it to fabricate metal cluster necklaces of unprecedented thinness and regularity.